1. Field of the Invention
The present invention relates generally to a color CRT (cathode-ray tube) apparatus.
2. Related Background Art
A color CRT apparatus forms color images by superposing images with three colors of R (red), G (green), and B (Blue) on a screen. For an imaging device, it is necessary to perform this superposition, i.e. convergence with high precision.
In a conventional color CRT, generally, three electron sources corresponding to the three colors R, G, and B are arranged in parallel, and angles of respective paths of electron beams on both sides of a center beam (herein after referred to simply as xe2x80x9cside electron beamsxe2x80x9d) with respect to a path of the center electron beam have been set so that the distance between becomes smaller gradually in the direction in which the electron beams travel, and imaging spots of the three electron beams emitted at an interval s between respective adjacent electron beams are superimposed on one point on the screen. In this case, since the distance from an electron gun to the periphery of the screen is longer than that from the electron gun to the center of the screen, when the angles are set so that the imaging spots of the three electron beams are superimposed on one point at the center of the screen, the conventional color CRT is designed so that the imaging spots of the three electron beams are superimposed on one point continually throughout the screen using means for easing the degrees (for reducing the angles) to which side electron beams are directed outward, according to the distance from the center of the screen (according to the deflection amount of the electron beams).
Such means are roughly divided into three specific means. The respective means are described briefly as follows.
A first specific means is called a xe2x80x9cself-convergence systemxe2x80x9d and basically is a method of providing nonuniformity to magnetic deflection field distribution. In a general example, a horizontal magnetic deflection field is provided with pincushion magnetic field distribution and a vertical magnetic deflection field is provided with barrel magnetic field distribution. The magnetic field distributions are designed so that imaging spots of three electron beams are superimposed on one point continually throughout the screen by causing a difference in deflection amount of the respective electron beams in the trajectory where the electron beams pass through the magnetic deflection fields while the intervals between respective adjacent electron beams of the three electron beams are the same at the moment they are emitted from an electron gun (for example, NHK (Japan Broadcasting Association) Color Television Textbook, Vol. 1, pages 267 to 271).
A second specific means is referred to as xe2x80x9cdynamic convergencexe2x80x9d. In the dynamic convergence, a magnetic field (a magnetic dynamic convergence field) for dynamically changing the angles of side electron beams with respect to the center electron beam is provided in the vicinity of main lenses of an electron gun and the strength of the magnetic field is varied according to an intended deflection amount, thus making adjustment so that imaging spots of the three electron beams are superimposed on one point continually throughout the screen (for example, NHK Color Television Textbook, Vol. 1, pages 266 to 267).
A third specific means employs a picture signal circuit (for instance, U.S. Pat. No. 2,764,628) constructed as follows. Three electron beams are not necessarily superimposed on one point on the screen, and side electron beams are set to be directed slightly inward or approximately in parallel. Spatial differences among images with three colors R, G, and B on the screen are corrected by temporal differences in input timings of modulating signals input to electron sources for the respective electron beams, thus allowing images formed of imaging spots of the three electron beams to be superimposed to be matched apparently throughout the screen. For convenience, such a means is referred to as xe2x80x9csignal phase convergencexe2x80x9d in the present specification.
Among those means, the first specific means has been used most widely. However, since the display density of image information has increased rapidly in recent years, it has become difficult to superimpose imaging spots of three electron beams with sufficiently high precision merely using the design of the above-mentioned magnetic field distributions. On the other hand, there are demands for reducing the depth of a color CRT apparatus. When the depth is to be reduced, the deflection angle increases. Therefore, the difference between the distance from the electron gun to the center of the screen and that from the electron gun to the periphery of the screen is increased further. Consequently, it has become more difficult to superimpose the imaging spots of three electron beams with high precision.
Therefore, the second specific means or a specific means of the combination of the first and second specific means has come to be used. In such a means, however, the three imaging spots are not always aligned on a straight line and are displaced irregularly. Therefore, a complicated system is required for correcting the irregular displacement, resulting in cost increase, which has been a disadvantage.
The third specific means basically achieves the convergence using a picture signal circuit. This is not a general technique, but the possibility of achieving this has increased with the improvement in digital circuit (for instance, JP 2542592 B). In this technique, the time difference corresponding to the displacement amount of the three imaging spots, which is different depending on positions on the screen, is applied to picture signals using a circuit technique, thus achieving the convergence throughout the screen. However, when this is achieved using the circuit technique alone, loads on the circuit increase, which include, for example, storage of the amounts of correction in signal phase at each position on the screen in a memory. Consequently, costs of the circuit and for its adjustment increase, which has been a disadvantage.
Some specific means of the combination of the first and third specific means also have been studied (for example, JP 54-29227 B and JP 6-46812 B). Basically, however, their designs are complicated, and naturally, there has been a limitation in coping with the increased display density and the increased deflection angle.
Of course, a specific means of the combination of the second and third specific means also can be considered as one combination. In this case, however, such a specific means comes to have both the disadvantages of the second and third specific means. In other words, such a specific means comes to have both the complicated correction system in the dynamic convergence and the increased load on the circuit due to the signal phase convergence. Therefore, such a specific means does not have much point and thus has not been studied.
The present invention is intended to cope sufficiently with the increased display density of image information and the increased deflection angle due to the reduction in depth by improving one obtained basically by combining elements of the above-mentioned second and third specific means and adding specific conditions thereto, by which images formed of imaging spots of three electron beams are superimposed on a screen with a sufficiently high precision to be matched without causing a cost increase.
In order to solve the above-mentioned problems, a first color CRT apparatus of the present invention employs the following system. In the system, three electron beams emitted from three electron sources arranged in an in-line form in a horizontal direction corresponding to three colors R, G, and B are deflected by a horizontal magnetic deflection field and a vertical magnetic deflection field, imaging spots of the three electron beams that are not superimposed on one point on a screen at a time are formed, and according to a time difference in timings at which the three electron beams scan the same one point on the screen, respectively, a time difference is applied to input timings of modulating signals for the respective electron beams to form color images on the screen. In the first color CRT apparatus, the screen is substantially flat, both the horizontal magnetic deflection field and the vertical magnetic deflection field have substantially uniform magnetic field distribution, respective paths of the side electron beams and a path of the center electron beam are approximately parallel to each other when the electron beams enter a magnetic deflection field area, and the time difference applied to the input timings of the modulating signals is approximately constant throughout the screen.
According to this, the images formed of imaging spots of the three electron beams are superimposed on the screen to be matched easily. Thus, it is possible to cope sufficiently with the increased display density of image information and the increased deflection angle due to the reduction in depth.
A second color CRT apparatus of the present invention employs the following system. In the system, three electron beams emitted from three electron sources arranged in an in-line form in a horizontal direction corresponding to three colors R, G, and B are deflected by a horizontal magnetic deflection field and a vertical magnetic deflection field, imaging spots of the three electron beams that are not superimposed on one point on a screen at a time are formed, and according to a time difference in timings at which the three electron beams scan the same one point on the screen, respectively, a time difference is applied to input timings of modulating signals for the respective electron beams to form color images on the screen. In the second color CRT apparatus, the screen is approximately flat, both the horizontal magnetic deflection field and the vertical magnetic deflection field have approximately uniform magnetic field distribution, and adjustment is made so that the imaging spots of the three electron beams on the screen at a time are positioned continually at a certain interval in a horizontal direction by allowing paths of the side electron beams and a path of the center electron beam to be approximately parallel to each other when the electron beams enter the magnetic deflection field area and the vertical magnetic deflection field is not generated and by increasing, according to strength of the vertical magnetic deflection field, intervals between the respective paths of the side electron beams and the path of the center electron beam when the electron beams enter the magnetic deflection field area and the vertical magnetic deflection field is generated.
According to this, the images formed of imaging spots of the three electron beams are superimposed on the screen with a sufficiently high precision to be matched without causing a cost increase. Thus, it is possible to cope sufficiently with the increased display density of image information and the increased deflection angle due to the reduction in depth.
In the color CRT apparatuses of the present invention, an electron gun may be designed so that the paths of the three electron beams inside the electron gun are approximately parallel to one another.
According to this, lens performance of the electron gun, particularly for the side beams can be improved.
Alternatively, even when the three electron beams inside the electron gun are not always parallel, a quadrupole magnetic field may be provided in the space between the screen and main lenses of the electron gun in the vicinity of the main lenses and may be adjusted statically so that the paths of the three electron beams entering the magnetic deflection field area are parallel to one another when the vertical magnetic deflection field is not generated.
In this case, a conventional electron gun can be used without any modification.
In this case, it is preferable that the quadrupole magnetic field includes a first quadrupole magnetic field and a second quadrupole magnetic field, which are provided sequentially in the space between the screen and the main lenses of the electron gun in the vicinity of the main lenses, the first quadrupole magnetic field bends the paths of the side electron beams of the three electron beams inward, and the second quadrupole magnetic field allows the paths of the three electron beams to be parallel to one another.
This prevents neck shadow from appearing easily.
In this case, further a third quadrupole magnetic field may be provided in the vicinity of the first quadrupole magnetic field or the second quadrupole magnetic field to bend the paths of the side electron beams inward or outward, or a quadrupole electrostatic lens may be provided inside the electron gun to bend the paths of the side electron beams inward or outward.
According to this, unbalance between the horizontal and vertical forces with respect to focusing action or lens magnification for imaging of the three electron beams can be corrected.
In the second color CRT apparatus of the present invention, it is preferable that a quadrupole magnetic field generated by electromagnetic coils is provided in the space between the screen and the main lenses of the electron gun in the vicinity of the main lenses and is adjusted dynamically so that when the vertical magnetic deflection field is generated, the intervals between the respective paths of the side electron beams and the path of the center electron beam entering the magnetic deflection field area are increased according to the strength of the vertical magnetic deflection field.
According to this, fine adjustment of the intervals between respective adjacent beams can be made so that the imaging spots of the three electron beams on the screen at a time can be positioned continually at a certain interval in the horizontal direction.
In the second color CRT apparatus of the present invention, it is preferable that a quadrupole magnetic field generated by electromagnetic coils is provided between the first quadrupole magnetic field and the second quadrupole magnetic field and is adjusted dynamically so that when the vertical magnetic deflection field is generated, the intervals between the respective paths of the side electron beams and the path of the center electron beam entering the magnetic deflection field area are increased according to the strength of the vertical magnetic deflection field.
According to this, besides the fine adjustment of the intervals between respective adjacent beams, the space between the first quadrupole magnetic field and the second quadrupole magnetic field can be set to be wide.
In the color CRT apparatuses of the present invention, it is preferable that raster distortion in the color images is corrected by the uniform magnetic field distribution or by an auxiliary magnetic field provided for the uniform magnetic field distribution in an auxiliary manner.
According to this, the raster distortion can be corrected without disturbing the convergence.
In the color CRT apparatuses of the present invention, it is preferable that the time difference in the input timings is adjusted according to variations in current level of the electron beams.
According to this, it is possible to make fine adjustment of the variations in intervals between respective adjacent imaging spots of the three electron beams on the screen caused by the occurrence of repulsion between electron beams (interelectronic repulsion) according to the variations in current value. Therefore, independent of the current value, the imaging spots of the three electron beams on the screen at a time can be positioned at a desired interval.
Furthermore, in the color CRT apparatuses of the present invention, geometric variations with the passage of time including those in size, linearity and shift of the raster in the color images may be detected with a sensor and the time difference in the input timings may be adjusted according to the amount of the variations.
According to this, the convergence can be prevented from being disturbed, even when the variations in size, linearity, and shift of the raster is caused with the passage of time.
In addition, in the color CRT apparatuses of the present invention, the time difference in the input timings may be adjusted according to settings of a field frequency and/or the number of pixels of display data of the color images.
According to this, the convergence can be prevented from being disturbed, even when the setting of the number of pixels of display data of the color images is changed.